In one conventional technique for the manufacture of semiconductor devices, a slice of semiconductor material (p or n-type) accepts a relatively thin layer, typically 5,000 to 10,000 A, of oxide grown on both of its surfaces. A layer of photoresist material is then spun on to the oxide of one side, and is subsequently exposed to UV light through a mask having openings corresponding to those areas on the semiconductor slice where it is desired to generate semiconductor junctions. After exposure of the photoresist material through the mask, the mask is removed and the layer of photoresist is developed and processed by means of a suitable solvent, exposing the underlying oxide layer. An acid dip is then used to etch the oxide from the surface of the semiconductor slice in the exposed areas, the remaining photoresist material serving as an etch-barrier for the oxide surface covered by it. Following the etching process, a water rinse and a drying cycle are implemented. The remainder of the photoresist material is subsequently removed, followed by an acid dip required for the removal of inorganic residues. Following a drying step, diffusion of dopant material into the exposed areas of the semiconductor slice (where there is no oxide) is commenced to produce a predetermined junction.
One of the problems associated with this particular technique arises from the step of removing the residual masking photoresist along with its inorganic contaminants prior to the diffusion stage. This step may be carried out by either one of two conventional techniques.
One technique employs a wet chemical process in which the residual photoresist is removed by application of a solvent. The solvent, however, does not simultaneously remove inorganic contaminants embedded in the photoresist material. These contaminants predominantly include tin, iron and magnesium metals, with much smaller traces of lead, copper, zinc, nickel, chromium, aluminum, calcium, titanium, sodium and manganese. Even in the purified versions of photoresist material, the tin concentration may be 130 parts per million, while that of iron and magnesium may be 5 to 10 parts per million. These contaminants, left on the oxide layer after the photoresist has been removed, cause faulty operation of the resultant semiconductor device. Consequently, an additional rather hazardous acid dip is required. There are a number of photoresist materials available in the market of which only a few have particularly desirable characteristics of adhesion and resistance to the etching acids. The product of Eastman Kodak Company identified as KMER is a case in point. As a result of its superior characteristics, excellent resolution and definition of the semiconductor junction areas may be achieved by its utilization. However, this photoresist material, even after purification, has a relatively significant level (&gt;100 parts per million) of contamination of tin. This tin residue exhibits undesirable effects as a contaminant on semiconductor and/or semiconductor oxide surfaces in terms of: (1) crystallization of the oxide and breakdown of its passivating properties leading to extraneous diffusions, (2) precipitations in the semiconductor acting as getters for the dopants leading to uncontrollable sheet resistivity, (3) induced charges in the oxide giving rise to electrical instabilities, junction leakage and uncontrollable drifts. Additional drawbacks of the wet chemical approach, involve the contamination of the solvents and their associated short shelf-life, as well as the continuing cost coupled with their use and the inconvenience in rinsing and drying procedures prior to the diffusion step.
A second technique, which has been employed, is a dry plasma stripping process in which the semiconductor device coated with the photoresist material is exposed to an oxygen discharge which degrades and volatilizes the organic photoresist material. This step, however, does not remove the inorganic materials embedded in the photoresist layer, but undesirably generates a very thin layer, of 50 to 75A of semiconductor oxide on the previously exposed portions of the semiconductor slice. This oxide layer is, of course, disadvantageous in the subsequent diffusion step, since it serves as a partial diffusion barrier for junction formation, and consequently an additional etching step to remove this oxide prior to diffusion is required.